Often, the ideological intransigence of Hamas is blamed for impeding Palestinian unity. But on Sunday it was the moderate Palestinian Authority president Mahmoud Abbas who dealt a potentially debilitating blow to the nascent Palestinian "unity" government  by appointing Mohammed Dahlan, the most divisive figure on the Palestinian political scene today, to the office of National Security Adviser.

The unity government was ratified on Saturday, in simultaneous proceedings in both Gaza and the West Bank connected via video conference technology to overcome Israeli prohibitions on most Palestinians' traveling between the two locations. Israel's response was predictably negative; officials pointed out that the new government had not accepted international demands to renounce violence and recognize Israel, and that even the moderates who now run key ministries are tainted by their political cohabitation with Hamas. Other governments showed more flexibility, ranging from full recognition of the new government (Norway, Arab countries) through cautious optimism (the European Union, Russia, the United Nations) to the idea that it might be possible to deal with ministers who were not members of Hamas (Britain, and even the United States).

The Palestinians have agreed to unite less out of common conviction than out of the necessity brought on by a crippling financial siege and Arab diplomatic pressure. The differences that persist among them were clear in the parliamentary session that ratified the new government: President Abbas made a speech emphasizing negotiations, while Prime Minister Ismail Haniyeh, Hamas' political leader, re-asserted the Palestinian right to "resistance," and spoke not of recognizing Israel, but of extending a truce if a Palestinian state were established along its 1967 borders.

But while those differences came as no surprise, Abbas's appointment of Dahlan, akin to throwing a grenade into an already tense room, threatens to shatter even the pretense of cooperation. Hamas immediately labeled the appointment "illegal," and urged Abbas to reconsider. The Palestinian Authority President could not possibly have been unaware of the impact his move would have. Dahlan, 45, currently a member of the Palestinian Legislative Council, was previously the boss of the Preventative Security office. He has extensive business interests in Gaza, and has been dogged for years by corruption allegations. A native Gazan from Khan Yunis, he was a leader of the first intifada and has spent time in Israeli jails. He later rose though the ranks of the PLO and took part in negotiations in Oslo in 1993 and at Camp David in 2000. Dahlan has close ties with the Americans and the Israelis, particularly their intelligence services.

Yuval Diskin, the head of Israel's Shin Bet intelligence service, said recently that he hoped to see Fatah rehabilitate its image and Dahlan, in particular, play a greater role. Predictably, then, he is loathed by Hamas members, whom he labeled "murderers" in January, and against whom he led a crackdown a decade ago. He also has enemies among some of the more powerful Palestinian clans, and was accused of playing a major role in the factional violence that convulsed Gaza in recent months.

"Dahlan by himself is a problem," says a Palestinian Foreign Ministry official, who wished to remain anonymous discussing sensitive issues, "but when you put him in such a position, he is more than a problem." A member of Fatah's Central Committee, who also preferred to go nameless on the topic of Dahlan, adds, "This is practically appointing him Interior Minister," the post responsible for overseeing the various security organizations, some of which are loyal to Fatah and some to Hamas. "This will bring us back to a bad situation. And it creates troubles for Haniyeh. He will have to oppose the move or face problems from inside Hamas."

Hamas had earlier vetoed Dahlan's nomination for the post of Deputy Prime Minister. Although his unilateral appointment as National Security Adviser move might reassure the Bush Administration, which has shown no enthusiasm for Abbas' joining Hamas in government, it will almost certainly deepen the wedge not only between Fatah and Hamas, but also between Abbas and the Palestinian public. "To the man and woman in the street, it seems that Abu Mazen [Abbas] is carrying out the orders of Israel and America," says the Fatah Central Committee member. Hamas legislators plan to oppose the move on the grounds that the Palestinian Basic Law forbids members of the legislature from serving in cabinet-level positions.

The furor comes as preparations begin for another visit to the region by Secretary of State Condoleezza Rice. She plans to meet with Abbas twice. If her aim was to stabilize Palestinian politics, she might ask Abbas what he was thinking. But given the dim view the Americans and Israelis hold of the unity government, and their positive view of Dahlan, she might also be tempted to congratulate him.